Added new flag (settable via a "exclude-range-header property) to indica...

...te if the HTTP Range header should be included in HTTP requests.  This is needed when extra headers are added which conflict with HTTP Range.  Also added "content-size" property which allow other elements to set the size of the content.  Setting this property to true will also make this content seekable.  Created new method "gst_soup_http_src_set_size()" to contain the necessary logic to be performed when content size is known or set via the content-size property.